Tunable coaxial cavity resonator with plunger mounted ring for shorting coupling loops



June 1, 1965 G. J. WHEELER 3,187,278

TUNABLE COAXIAL CAVITY RESONATOR WITH PLUNGER MOUNTED RING FOR SHORTING COUPLING LOOPS Filed Nov. 12, 1963 I l 3\ /I5 2/ A 20 i '4 I IO u eERsHonN. WHEELER T-TIET-ZI BY Awf%,./&-

ATTORNEY United States Patent 3,187,278 TUNABLE COAXIAL \CAVHTY RENATOR WITH PLUNGER MOUNTED G FOR SHURTHIG COUPLTNG LOOPS Gershon 3. Wheeler, Los Altos, Calii, assignor to ylvania Electric Products Inc, a corporation of Delaware Filed Nov. 12, 1963, Ser. No. 322,711

1 Claim. (Cl. 33382) This invention relates to tunable cavity resonators, and more particularly to an improved tunable coaxial-cavity resonator.

A coaxial-cavity resonator may be tuned by moving a conducting plunger within the cavity in such a manner that the plunger provides a short circuit between the outer and inner conductors as it moves. Disadvantages of this tuning technique are losses, erratic operation due to sliding contacts and Wear of the sliding parts, and the size of the device.

In accordance with my invent-ion, a coaxial-cavity is tuned with a dielectric element that is moved axially within the cavity between the inner and outer conductors. More specifically, a coaxial-cavity having an effective length equal to a quarter wavelength'at its highest operating frequency and being open at one end is provided with a tubular dielectric plunger which is movable within the cavity toward the closed end to decrease the resonant frequency of the cavity. Coaxial coupler-s near the closed end of the cavity couple energy to and from the cavity. A modified embodiment features a conducting ring on the inner end of the dielectric plunger which functions as an integral switch to short out the cavity when the plunger engages the input and output couplers at the inner limit of the stroke.

An object of the invention is the provision of a compact tunable cavity which overcomes the disadvantages of sliding metallic contacts or choke joints.

Another object is the provision of a tunable cavity whose physical length decreases as its operating frequency decreases.

Another object is the provision of a tunable cavity with an integral switch which automatically electrically removes the cavity from associated circuits at the limit of the tuning stroke.

These and other objects of the invention will become apparent from the following description of preferred embodiments thereof, reference being had to the accompanying drawings in which:

FIGURE 1 is a side view, partially in section, of a single tunable coaxial-cavity embodying the invention;

FIGURE 2 is a longitudinal section of a dual cavity tunable resonator embodying this invention; and

FIGURE '3 is a side view partly in section of the tunable cavity of FIGURE 1 modified to include a switch for electrically isolating the cavity from the associated circuit.

Particular reference being had to FIGURE 1, the resonator comprises a coaxial cavity 1 having a tubular dielectric plunger 2 movable therein and an input coupler 3 and output coupler 4. The cavity is preferably a quarter wavelength coaxial-cavity and comprises a cylindrical outer conductor 5 and an inner conductor 6. The surfaces of the conductors in the cavity may be silverplated to reduce the insertion loss. The plunger has a central opening 7 within which the center conductor 6 fits when the plunger is moved over the center conductor.

The length of outer conductor 5 is preferably greater than a quarter wavelength at the highest operating frequency for supporting plunger 2 in the cavity. The cavity is closed at one end by conducting closure 8 which provides a short circuit between the inner and outer conductors, and is open at the other end 9 to provide an open circuit there. The inside diameter of outer conductor 5 is sufficiently small so that the resonant frequency of the cavity is below cutoff for the dominant TE cylindrical mode, even when. the cavity is loaded with dielectric, to prevent propagation of a signal out the open end of the cavity and to prevent excitation of higher order coaxial modes.- In accordance with known cavity design practice, inner conductor a is shorter than outer conductor 5 for a quarter wavelength cavity. A fringing electric field develops between the conductors from the end of conductor 6 to plane A-A so that the effective length l of inner conductor 6 is equal to the axial spacing of closure 8 and plane A--A. The effective length of inner conductor 6 determines the highest resonant frequency of the cavity when the plunger 2 is withdrawn to or beyond plane A-A. The quarter wavelength cavity is measured by the axial spacing of closure 8 and plane AA.

The dielectric constant of movable plunger 2 is selected in accordance with the operational requirements of the system as will be discussed more fully hereinafter. The outer diameter of plunger 2 is a few thousandths of an inch less than the inner diameter of outer conductor 5 to prevent binding between sliding surfaces while preventing undue radial play between the parts. The inner diameter of the plunger is slightly larger than the diameter of inner conductor 6 for the same reason. Plunger 2 is connected by a shaft 10 to drive means 11 which moves the plunger axially within the cavity. Drive means 11 may be a servomechanisrn which is responsive to electrical or mechanical control signals for adjusting the resonant frequency of cavity 1.

Input and output couplers 3 and 4 comprise inner conductors 13 and .114 and outer conductors '15 and 16, respectively. loops 1'7 and 18 within the cavity and are connectedat their ends to the outer conductor 5 at 19 and 20, respectively. Coupler outer conductors 15 and 16 are also electrically connected to cavity outer conductor 5.

The electrical length of a transmission line filled with dielectn'c is increased by the square root of the dielectric constant of the dielectric in the line. If the dielectric plunger 2 is removed from the coaxial cavity 1 to the plane AA, the electrical and physical length of the quarter wavelength cavity are equal and where 6 is electrical length of the cavity and l is the physical length of the cavity. If the plunger is moved into the cavity a distance x as shown in FIGURE 1, the physical length of the cavity is still I, but its effective electrical length is where e is the dielectric constant of the plunger. Thus, if the plunger has a dielectric constant greater than one, the electrical length of the cavity is increased by 1)x, Equation 3, and the resonant frequency of the cavity is decreased. The electrical length and resonant frequency of the cavity are directly related to the position of the plunger in the cavity. The cavity is therefore tuned by change of position of the dielectric plunger in the cavity by drive means 11.

By Way of example, two tunable cavities having the following dimensions and characteristics were constructed and successfully operated:

Cavity:

Outer conductor 7 Length "inches" 1.5 Inside diameter do 0.520 Inside conductor Length do 0.75 Diameter do 0.150

Patented dune 1, 1965 l Inner conductors 13 and Marc bent into Plunger (first .type) Outside diameter do 0.500 Opening diameter do 0.190 Material Quartz 6=3-99) Plunger (second type) Outside diameter inches 0.515 Opening diameter do 0.155 Material Stycast K-6 (6:6)

Plunger Plunger (First Type) (Second Tuning Range .ine 2, 200-3,540 1, 700-3, 540 Insertion Loss db 0. 6 0. 4 0. .4 Bandwidth at 3 db me 28-31 28-32 In order to sharpen selectivity, a dual cavity tunable resonator using two improved cavities as shown in FIG- URE 2 is provided. The resonator comprises a rectangular block 22 having two cylindrical cavities 23 and 24 formed therein on parallel axes to a depth less than the full length of the block. The cavities are defined by outer block walls 25 and 26 and a common center wall 27; They are closed atone end by end wall 28 and are open at the opposite end.

Center conductors 29 and 30 extend coaxially into the cavities, respectively, from wall 28 for a distance less than the full length of the block 22 and preferably comprise conducting rods press fitted into suitable openings 31 and 32, respectively, in end wall 28. Block 22, including walls 25, 26, and 2-7, constitutes the outer conductor for each of the coaxial cavity resonators.

Input coupler 34 and output coupler 35 extend into the respective cavities through walls 25 and 26, respectively,

and are aligned with each other and With a coupling hole or iris 36 in center wall 27 which couples the cavities together. Thedesign and position of the loops 34 and 35' of the respective couplers and of the iris 36 are described in sections 28-48, 28-25, and 32-25 of VHF Techniques, volume 2, RRL Series, Harvard University.

modified to incorporate a switch for electrically isolating the cavity firom associated circuitry; The outer diameter of a short length of plunger .2 is reduced at its inner end 42, and a ring 43 of conductive material spaced inward.- ly from outer conductor is secured to the face of that reduced end. When plunger 2 is moved to its low frequency limit, illustrated by the dotted lines to the left in FIGURE 3, conductive ring 43 contacts and short circuits coupler inner conductors 13 and 14 and electrically removes the cavity from the circuit to which the couplers are connected. Thus the tunable cavity has a built-in isolation switch.

Although this invention has :been shown and described in the relation to preferred embodiments thereof, variations and modifications will be apparent to those skilled in the art. For example, the basic invention disclosed herein is also applicable toother types of resonant devices such as a half wavelength coaxial-cavity resonator. The scope and breadth of this invention is, therefore, to be determined from the following claim rather than from the above detailed disclosure.

What is claimed is:

A tunable cavity resonator comprising a quarter wavelength coaxial-cavity having one end closed and the other end open and comprising an outer conductor, an inner conductor, and a conductive member extending transversely of and electrically connecting said conductors at said closed end of the cavity,

input and output coupling loops for transferring energy between said cavity and external associated circuits,

a dielectric plunger extending into said cavity through said open end and having a dielectric constant greater than one, a a conductive strip secured to the end of said plunger facing said loops and electrically insulated from said conductors,

said strip being engageable with said loops at one limit of plunger movement for short eircui-ting said loops whereby to electrically isolate said cavity from said external circuits, and means formoving saiddielectric plunger longitudinally in said cavity whereby .to change the electrical length of said cavity to vary the resonant frequency of said cavity.

References Cited by the Examiner UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Examiner. 

